I had the same problem with my Insprion 8100 with ESS Maestro-3i sound. The notifications were working when I installed Kubuntu 5.1, but when I upgraded KDE to 3.5.1 they stopped working. The solution for my system was really simple. In System Settings, go to Sound & Multimedia, Sound System, then the Hardware tab. Select an audio device other than "Autodetect". In my case "Advanced Linux Sound Architecture" seems to work really well.